Research groups at IBM and other labs have developed methods to form multi-layer films composed of a base ferromagnet (like iron or permalloy), whose spin orientation is fixed in a given direction, a second layer of an ordinary metal (such as copper), and a top magnetic layer whose spin orientation can be rotated by an external magnetic field. These structures exhibit a giant magneto-resistance (GMR), which is used in magnetic storage of computer memory. The method measures changes in electrical current and voltage to detect the external magnetic field.
Prior studies of magneto-optical recording techniques employ the Kerr or Faraday effects to rotate the light polarization by the magnetic forces in a material. The standard Kerr technology may employ a single magnetic film (e.g., cobalt) deposited on a metal like copper, and such technologies are disclosed in xe2x80x9cMagnetic Recording Technologyxe2x80x9d, C. Denis Mee and Eric D. Daniel, editors, chapter 10 (McGraw-Hill, NY 1995).
The tri-layer arrangement of a simple metal sandwiched between two magnetic layers is called a xe2x80x9cspin-valvexe2x80x9d system, because an external magnetic field controls the flow of electrical current by regulating the scattering gate for electrons with a fixed spin direction. The corresponding giant magneto-resistance (GMR) in Fe/Cr/Fe multi-layers was discovered by M. N. Baibitch et al, Phys. Rev. Lett. 61,2472(1988). The application of GMR devices for computer storage has stimulated enormous interest, with more than 1,000 papers published on electrical resistance, magnetic, and structural properties of such films. However, the magneto-optic modulation and its relationship to reflectivity has not been adequately explored.
Certain metals exhibit a dramatic change in their optical reflectivity, which is caused by electron collisions between nearly parallel paths. Such electronic trajectories are said to be xe2x80x9cnestedxe2x80x9d and tend to favor anti-ferromagnetic spin alignment in metals like chromium. Thus, the unconventional optical conductivity features, which have been observed in chromium, rare earth metals, and in high temperature superconductors, may be used in accordance with the present invention to detect a magnetic field.
The magneto-optic device may also be used as an optical switch or light modulator for fiber-optics telecommunications. A change in the transmission and reflectance of infrared light through the multi-layer film can be controlled by a small external magnetic field. Thus the film can encode a signal on the carrier light beam, or reflect the light beam in a desired direction for purposes of switching signal paths. The spin-flip in the magnetic layer occurs in less than one nanosecond, so that the optical switch application would yield very high bandwidth capabilities.
Silicon substrates for the metal films are preferable for fiber-optic applications. The thickness of the inner layer metal film is chosen to yield an anti-ferromagnetic coupling between the top and bottom magnetic films. The external field will overcome this weak tendency toward anti-parallel spin alignment and rotate the spins to point in the same direction. The resulting exchange forces modify the electronic structure in the middle layer and thus alter its ability to reflect or transmit light.
The theoretical background for the magneto-optic sensor invention stems from calculations of the optical reflectivity of high temperature superconductors. A review of this theory basisxe2x80x94in the absence of magnetic field effectsxe2x80x94is disclosed in J. Ruvalds, Superconductor Science and Technology 9,905 (1996).
Optical switching in telecommunications currently converts light signals to electrical current in circuits, processes the information by relatively slow electronic methods, and then converts the message to another infrared light beam. Since this electronic delay limits bandwidth for internet communications, there is interest in switching a light beam by purely optical methods. Examples of new optical switches include xe2x80x9cbubblesxe2x80x9d, xe2x80x9cmicro-mirrorsxe2x80x9d, and liquid crystals whose merits and promise are described in: MIT Technology Review, July/August 2000.
The present invention provides a layered arrangement of metal films, where the inner layer exhibits a large change in the optical reflectivity as a function of frequency of the incoming light. A standard Drude reflectance of ordinary metals (like lead, copper, or silver) remains close to 95% over a wide range of incoming light energies that extend to 1 eV or more, as shown in FIG. 1. However, a dramatic drop of the reflectivity (to about 20% near 1 eV) has been calculated for electron collisions on nearly parallel trajectories, and the application of a magnetic field should restore the latter anomalous small reflectance toward the normal high value of a typical metal. Thus, a desirable metallic inner layer in the magneto-optic sensor should have a spectrum that resembles the computed solid curve in FIG. 1. Materials that exhibit the anomalous reflectivity drop include chromium, rare earth metals, and high temperature superconductors.
An external magnetic field will split the electron energy levels in the inner layer metal and thus remove the origin of the anomalous reflectivity. Hence, a change in the optical reflectance in the visible light range will be a particularly good indicator of a magnetic field. At microwave and infrared frequencies, smaller changes in the inner layer reflectivity may also be used to detect a magnetic field.
The base layer of a ferromagnetic metal (like iron or permalloy) amplifies the effect of an external field, which rotates the spin orientation of the top magnetic layer. FIG. 2 illustrates parallel spin alignment in a multi-layer structure that tends to maximize the spin splitting of the energy bands in the inner layer. This type of multi-layer structure produces a giant magneto-resistance (GMR) for an ordinary inner layer metal like copper, and such DC resistance devices are used for computer memory storage. However, the design of a multi-layer device for the purpose of detecting a magnetic field by measuring the infrared or optical reflectivity is new.
Transmission of light through a multi-layer film can be modulated by a small external magnetic field via very strong exchange forces between a magnetic layer and an adjacent layer of a metal like chromium. When the external field aligns the magnetic spins of top and bottom layers, the electron motion in the inner layer is modified by the exchange coupling to the adjacent magnetic layers. In the case of chromium, the applied magnetic field may change the infrared light transmission from 40% to 5% or less. Hence the multi-layer can be used to switch or modulate a light beam in a silicon fiber that is used for data or voice transmission.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings.